f(x+y,x-y)=x^2-y^2=(x+y)(x-y)令x+y=a，x-y=b那么f(a,b)=ab所以f(x,y)=xy
已知，f(x+y,x-y)=x²-y²=(x+y)(x-y)所以，f(x,y)=xy
设 u=x+y,v=x-yf(u,v)=f(x+y,x-y)=x^2-y^2=(x+y)(x-y)=uv得f(x,y)=xy
f（x，y）=xy
